Nacelle Testing and Examination of Electrical Characteristics

Speeding up Time-to-Market with Testing for Grid Integration

The expectations placed on the performance of wind turbines have increased significantly during recent years. The increasing competitive pressure which prevails on the global market and the noticeable professionalization of the industry have increased these expectations: With new turbine designs, the expectation nowadays is that the first turbines of a new type already run with high reliability when they are first delivered. Investors demand proof of comprehensive operational experience before they will commit financing for projects. New developments – even modifications of existing products – therefore represent a significant economic risk as far as the manufacturers are concerned. The experimental validation of prototypes on large test benches reduces this risk, accelerates the certification, and improves the plannability.

The higher proportion of electricity from regenerative sources in the distribution and transmission grid structures at various voltage levels increases the demands being placed on the grid integration of wind turbines as power generating units (PGU) even further. These requirements are laid down in standards and guidelines which have to be taken into account in future developments. Turbine certificates are mandatory for new and modified turbine designs. They ensure that the PGU operation is compliant with the grid code and thus guarantee the grid connection in the long term and the continuation of the feed-in tariff.

Fraunhofer IWES assists turbine manufacturers by offering efficient test methods for the accelerated validation of the electrical properties of PGUs on test benches to meet the increasing requirements.

Status quo: field test and limited predictability for market launch

The testing of grid compatibility for the certification of the electrical characteristics of new wind turbines – or recertification when existing types of turbines are modified or improved – is currently undertaken almost exclusively with the aid of mobile test installations in the field. To determine the electrical characteristics, these field tests always include the following measurements, which are taken with the aid of the test installation, Fault-Ride-Through (FRT) containers and measuring systems.

The complete certification campaign usually covers a period of up to two years; this amount of time is a significant cost factor in turbine development and decisively determines the point in time at which the turbine is launched commercially. The demand for suitable locations for the prototype certification is high, as is the number of turbines to be certified. The site conditions largely determine the realizability of a certification campaign: alongside the need for good wind conditions, increasing turbine size means that higher demands are placed on the grid connection. Test for the certification of the electrical characteristics of power generating units affect the downstream supply network and therefore require close coordination in advance with the local network operator. In addition, field tests are practically irreproducible; it is extremely unlikely that two tests can be performed under precisely the same wind and grid conditions. The comparability of the results for verification is therefore limited. Moreover, there can sometimes be long delays before the requisite test conditions can be met. This therefore makes it much more difficult to plan the series production for the commercial launch.

Advantage of laboratory testing: reproducing critical load cases at a random number

In 2015, a first nacelle test rig for complete nacelles has been put into operation. Fraunhofer´s Dynamic Nacelle Testing Laboratory (DyNaLab) provides turbine manufacturers with a realistic testing environment in the multi-megawatt range to carry out tests under reproducible conditions. This combination of mechanical tests and a grid emulator to test wind turbines up to 10 MW offers unique possibilities for prototype validation. By using an artificial network with 44 MVA installed converter power, it is possible to reproduce typical grid faults such as voltage dips with a high repetition rate. In the Hardware-in-the-Loop (HiL) method, high-performance, real-time models and corresponding control algorithms are used to operate the test bench including the unit under test.

Compared with a field based certification campaign, DyNaLab shortens the certification process significantly: certain critical load cases which might appear during operation can be reproduced at a random number. A testing campaign for certification on the test bench can be scheduled precisely and defined so as to be manufacturer specific. In this manner, operation management and control can be improved and models can be validated. Therefore, reliability and availability of the turbine can be improved, and costs for maintenance and repair cut down.

Safeguarding new and modified turbine designs

The Dynamic Nacelle Testing Laboratory (DyNaLab) provides turbine manufacturers with a realistic testing environment in the multi-megawatt range to carry out tests under reproducible conditions within a specified time period. Existing and future concepts for wind turbines can thus be validated and optimized where necessary. By using an artificial network with 44 MVA installed converter power, it is possible to reproduce typical grid faults such as voltage dips with a high repetition rate.

This combination of mechanical tests and a grid emulator to test wind turbines up to 10 MW is currently the only one anywhere in the world. Since it was commissioned in 2015, the prototype of AD 8-180 was tested, and Enercon and Siemens Gamesa Renewable Energy have used the nacelle test bench for their campaigns. Moreover, a superconducting generator was tested at the facility as part of the EcoSwing research project.

The high-performance grid emulator allows static tests to be carried out to determine the effective and reactive power output for different grid conditions, for example. In addition, transient grid events which affect the whole nacelle system can be simulated: Tests of dynamic Under-Voltage-Ride-Through (UVRT) and High-Voltage-Ride-Through (HVRT) events, as are demanded by various grid codes, and dynamic changes to the grid frequency can be specifically reproduced and their effects on the turbine analyzed.

Since the nacelle is tested on the test bench without rotor and tower, it has different system characteristics than it has in the field. To replicate the actual conditions, the loads and interactions which occur between nacelle and rotor are calculated and imposed on the nacelle on the test bench. In the Hardware-in-the-Loop (HiL) method, high-performance, real-time models and corresponding control algorithms are used to operate the test bench including the unit under test. A testing campaign for certification on the test bench can be scheduled precisely and defined so as to be manufacturer specific.

Fraunhofer IWES is satisfying the sustained demand from the industry for accelerated testing possibilities for the electrical system of a wind turbine by building a new test bench which will go into operation in 2019. Unlike the nacelle test bench, this one will be designed to test minimal systems. They consist of high-speed generators and converter systems, as well as components for grid integration on the medium voltage level. Therefore, the test bench is appropriate for systems with high generator numbers of rotation (1200-1800 1/min) up to 6-7 MW nominal power with two- to three-stage gearboxes.

The aim is for test methods being developed as part of HiL-GridCoP, a BMWi supported project, to facilitate the partial automation of the processes necessary for the certification of the electrical characteristics. Turbine manufacturers thus save time and money on the testing process and also because the logistics are simpler. The first companies to use the test bench will be the HiL-GridCoP project partners Senvion, Nordex, and Vestas.

To test all the characteristics of a wind turbine in the laboratory, development tests with several different runs are necessary and their execution is to be partly automated. The software-assisted test management utilizes approaches from the automobile industry to provide the client with standardized interfaces, generate test profiles from the test specification, and allow the tests to be carried out with only partial manual control.

The test bench will have a 9 MW drive unit (up to 13 MW in overload) to replicate realistic generator moments with the aid of Hardware-in-the- Loop (HiL) techniques. The demands being placed on the HiL methods of testing are increasing – apart from replicating the rotor on the nacelle test bench, they must also be able to emulate the drive train. This particular demand is met by using detailed, real-time models on high-performance target hardware which allows optimum data exchange with the test bench control.

The test bench uses the existing grid emulation of the nacelle test bench to replicate various grid conditions. IWES also plans to expand the functionalities of the existing grid emulators significantly so as to meet the demands for the grid integration of future wind turbines as well. The aim is to be able to replicate systems from extremely weak grids up to special harmonic interference spectra.

Ever-increasing requirements are being placed on the grid integration to ensure the distribution and transmission network operates with high stability on the different voltage levels as the proportion of fluctuating feed-ins increases. This is evident, for example, in the increasing requirements being placed on the main converters of wind turbines in respect of the electrical characteristics of their power quality (PQ) - regardless of their drive train topology. A logical next step is to reduce the system being tested further to the (main) component: the converter.

Analyses relating to power quality and the certification of the electrical characteristics of the converter are already state of the art for generation and consumer units in the low voltage category (several kW). It is against this background that Fraunhofer IWES is planning to develop concepts for testing and validating converter systems further and thus provide this branch of industry with new ideas. These activities furthermore aim to strengthen and expand the competitiveness of the converter manufacturers as important component suppliers for the wind power industry.